如何使用 TabLayout 将数据从 Activity 传递到 Fragment

Question

I have been trying to pass data from Main Activity to fragments using bundle. I have used TabLayout and ViewPager to add fragments on my activity in Tab format. I don't get any error but I get a null object in return and it displays "ERROR" on my 1st fragment as I have added an If condition for null object. This is my code

Main Activity Code

package com.example.activitytofragmenttest;

import androidx.appcompat.app.AppCompatActivity;
import androidx.viewpager.widget.ViewPager;

import android.os.Bundle;
import android.widget.TextView;

import com.google.android.material.tabs.TabLayout;

public class MainActivity extends AppCompatActivity {

    TextView textView;
    private ViewPager viewPager;
    private ViewPagerAdapter viewPagerAdapter;
    private TabLayout tabLayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //Text View
        textView = findViewById(R.id.mainTextView);

        //TabLayout and ViewPager
        tabLayout = findViewById(R.id.tabLayout);
        viewPager = findViewById(R.id.viewPager);
        viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());
        tabViewPager();

        Bundle bundle = new Bundle();
        String message = textView.getText().toString();
        bundle.putString("key",message);
        Fragment_1 fragment_1 = new Fragment_1();
        fragment_1.setArguments(bundle);

        viewPager.setAdapter(viewPagerAdapter);
        tabLayout.setupWithViewPager(viewPager);
    }

    private void tabViewPager() {
        viewPagerAdapter.AddFragment(new Fragment_1(),"Frag1");
        viewPagerAdapter.AddFragment(new Fragment_2(),"Frag2");
        viewPagerAdapter.AddFragment(new Fragment_3(),"Frag3");
    }
}

ViewPager Code

package com.example.activitytofragmenttest;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentPagerAdapter;

import java.util.ArrayList;
import java.util.List;

public class ViewPagerAdapter extends FragmentPagerAdapter {
    private final List<Fragment> listFragment = new ArrayList<>();
    private final List<String> listTitle = new ArrayList<>();


    public ViewPagerAdapter(@NonNull FragmentManager fm) {
        super(fm);
    }

    @NonNull
    @Override
    public Fragment getItem(int position) {
        return listFragment.get(position);
    }

    @Override
    public int getCount() {
        return listTitle.size();
    }

    @Nullable
    @Override
    public CharSequence getPageTitle(int position) {
        return listTitle.get(position);
    }


    public void AddFragment (Fragment fragment, String title){
        listFragment.add(fragment);
        listTitle.add(title);
    }
}

Fragment 1 Code

package com.example.activitytofragmenttest;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;

public class Fragment_1 extends Fragment{

    TextView fragment_1TextView;

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_1,container,false);

        fragment_1TextView = v.findViewById(R.id.fragment1TextView);
        Bundle message = getArguments();
        if (message != null){
            fragment_1TextView.setText(message.getString("key"));
        }else {
            fragment_1TextView.setText("Error");
        }
        return v;
    }
}

Fragment 2 Code

package com.example.activitytofragmenttest;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;

public class Fragment_2 extends Fragment {
    TextView fragment_2TextView;

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_2, container, false);

        fragment_2TextView = v.findViewById(R.id.fragment2TextView);
//        Bundle bundle = getArguments();
//        if (bundle != null) {
//            fragment_2TextView.setText(bundle.getString("key"));
//        }
        return v;
    }
}

Fragment 3 Code

package com.example.activitytofragmenttest;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;

public class Fragment_3 extends Fragment {
    TextView fragment_3TextView;

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_3, container, false);

        fragment_3TextView = v.findViewById(R.id.fragment3TextView);
//        Bundle bundle = getArguments();
//        if (bundle != null) {
//            fragment_2TextView.setText(bundle.getString("key"));
//        }
        return v;
    }
}

Can anyone suggest why I get a null object from the Main Activity to fragment, and how can I transfer multiple data from the Main Activity to other fragments as well that is 2nd and 3rd fragment.

Answer 1

When starting the Fragment you can pass parameters through a bundle

/*ACTIVITY CODE*/
Bundle args = new Bundle();
args.putStringArrayList("argument_name", myListInString);
MyFragment fragment = new MyFragment();
fragment.setArguments(args);
fragment.show(fragmentTransaction, "fragment_tag")

And use them in your new snippet as follows

/* FRAGMENT CODE */
ArrayList<String> myList = getArguments.getStringArrayList("argument_name");
// ... FILL TABLE

Answer 2

This is a code ordering and scope issue.

You create a new Fragment_1 in

viewPagerAdapter.AddFragment(new Fragment_1(),"Frag1");

and store it in the ViewPagerAdapter , this has no Bundle attached.

You then later create a new instance of Fragment_1 in the local scope of of oncreate and attach the Bundle to it and then throw this instance away.

A better ordering of the code.

        //....
        viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());

        Bundle bundle = new Bundle();
        String message = textView.getText().toString();
        bundle.putString("key",message);
        Fragment_1 fragment_1 = new Fragment_1();
        fragment_1.setArguments(bundle);

        // Add the existing instance of the Fragment_1 to the viewpager instead of creating another new instance.
        viewPagerAdapter.AddFragment(fragment_1,"Frag1");
        viewPagerAdapter.AddFragment(new Fragment_2(),"Frag2");
        viewPagerAdapter.AddFragment(new Fragment_3(),"Frag3");

        viewPager.setAdapter(viewPagerAdapter);
        //....